The invention relates to a method of manufacturing a cathode ray tube, said cathode ray tube comprising a funnel-shaped part connected to a neck part, the funnel-shaped part having an inner surface and a high-voltage connection element, the method comprising the steps of: arranging the tube with its neck part in a downward position, covering the inner surface of the funnel-shaped part with a carrier liquid comprising a conductive material, the covering step comprising: supplying the carrier liquid through a pipe positioned near an edge of the funnel-shaped part and close to the inner surface of the funnel-shaped part, the pipe and the funnel-shaped part being moved with respect to each other, a projection of the pipe on the inner surface thereby describing a trajectory on the inner surface of the funnel-shaped part which is substantially parallel to the edge of the funnel-shaped part, and allowing the carrier liquid to drain downwardly, thereby leaving a coating formed by a residue of the conductive material on the inner surface. The invention also relates to a device for manufacturing a cathode ray tube
U.S. Pat. No. 4,151,312, issued on Apr. 24, 1979, describes a method of manufacturing a cathode ray tube (CRT) by providing an electrically conductive coating on an inner surface of a funnel-shaped part of the tube. A suspension comprising a conductive material flows from a container through a pipe which discharges on the inner surface of the funnel-shaped part and hits said surface approximately below an edge of the funnel-shaped part. The pipe is moved along the edge of the funnel-shaped part until the inner surface is covered everywhere with a uniformly thick layer of the suspension. The wet suspension is subsequently dried.
This way of providing the coating has the disadvantage that also the high-voltage connection element comprised on the funnel-shaped part becomes completely covered with the coating. After the coating step, a so-called getter spring is attached to the high-voltage connection element. If the high-voltage connection element and an area around it are not completely clean, then this assembling step may introduce unwanted loose particles from the coating into the tube. Consequently, the high-voltage connection element has to be cleaned prior to the connection of the getter spring. This requires an additional process step. Such a cleaning step has to be done extremely well to prevent loose particles from being further introduced into the tube.
It is an object of the invention to provide a method and a device for manufacturing a CRT that does not require an additional cleaning step after the coating step. To this end, the invention is characterized in that, during the covering step, the pipe and the funnel-shaped part are moved with respect to each other to obtain a which is a non-closed loop, such that the high-voltage connection element is not completely provided with the coating.
This way of providing the coating leaves the high-voltage element uncovered so that no further cleaning steps are required. As a consequence, no loose particles from a coating area around the high-voltage connection element can be generated. Thus, the manufacturing yield of the tube is improved and the cost price of the product is reduced.
Advantageous embodiments of the invention are defined in the dependent claims.
It is to be noted that JP-A 3-71542, published on Mar. 27, 1991, discloses a cathode ray tube, which is provided with a conductive film that is applied to the inner wall surface of a funnel, except for an inner portion of an anode button terminal provided on the inner wall surface, a nipple portion and near an inner portion of the terminal. Thus, a high-voltage applied to the terminal is transmitted and supplied to the whole of the film by the contact of a getter supporting metal plate, the plate extending from the nipple portion. The film is prevented from peeling off due to the contact and vibration of the junction point of the nipple point with the metal plate and the withstand property of the cathode ray tube is enhanced so that a stable image is obtained. However, this document is silent on how the conductive film is applied in such a manner that the inner portion of the anode button terminal is excepted.